1. Field of the Invention
This invention relates to a tubular reactor integrated with an heat exchange apparatus particularly well suited for removing large amounts of heat from the reactor at essentially isothermal conditions.
2. Background of the Invention
A variety of commercially important chemical reactions, and in particular catalytic vapor-phase partial oxidation reactions, are performed in tubular reactors in which maintaining the temperature of the reactants within a narrow temperature range is critical for achieving desired product yield, selectivity and properties. Many of these reactions are fast and highly exothermic. Such reactions are conventionally conducted at elevated temperature in heterogeneous catalytic tubular reactors. Most often such reactors are built as shell and tube heat exchangers in which the tubes are filled with an appropriate catalyst supported on a porous medium. Gas reactants are fed into the reactor tubes where they flow past the catalyst and react to form the desired product. The heat of reaction is quickly transferred from the site of the reaction to the outside walls of the tubes. A circulating or boiling coolant on the shell side removes the heat of reaction. Conventionally, the oxygen content of the reactants is kept low to stay outside of the explosive limit of the hydrocarbon—oxygen mixture and to minimize the formation of the complete oxidation by-product, carbon dioxide.
Two methods of cooling are customarily used with tubular reactors, forced circulation or the boiling of a suitable heat transfer fluid. Poor heat transfer between the reactor tubes and the heat transfer fluid often results in an undesirable temperature profile along the reactor tube length. Typically, the temperature profile of an exothermic catalytic reaction is low at the feed end, rises to a maximum at a central tube section and then drops off as the reaction is starved of reactants. Thus, vapor-phase fixed-bed tubular catalytic reactors exhibit a pronounced and undesirable temperature “hump” or the “hot spot”.
Reactor temperatures which exceeds the optimum temperatures for a given reaction result in lower selectivity as undesirable products are formed. Undesirable side reactions also lead to localized high temperature hot spots which can carbonize organic heat transfer fluids. One reaction in which this can occur is the highly exothermic partial oxidation of ethylene to ethylene oxide where the heat transfer fluid is typically DOWTHERM or tetrahydronaphtalene or similar HTF.
The temperature profile of a catalytic tubular reactor often changes with time as the catalyst deactivates. In general, a catalytic tubular reactor's hot spot tends to move from its inlet end to its outlet end. Several methods have been proposed to control hot spots in catalytic tubular reactors, but all are deemed to be undesirable as either impractical or because they have the undesirable side effect of decreasing selectivity. Examples of such prior art methods are:    Gelbein (U.S. Pat. No. 4,261,899) proposed the use a dilute phase transported-bed (riser reactor) with a variable diameter and a fluidized-bed heat exchanger.    M. Yoshida and S. Matsumoto, (Journal of Chemical Engineering of Japan, Vol. 31, No. 3, pp. 381-390, 1998) suggested a single tube reactor with multiple electrical heaters.    A. I. Anastasov and V. A.Nikolov, (Industrial. Eng. Chem. Research. No. 37, pp. 3424-3433, 1998) disclose the use of dual reactors in series to optimize the overall performance.    Patent DE 3,935,030, JA 60-7929 and EP 339,748 describe reactors where cooling coils are embedded within the catalyst bed. This solution is impractical because of the very small tube size (between 20-40 mm) required for heat transfer.    In U.S. Pat. No. 5,262,551 methane is employed as a ballast gas instead of nitrogen. Methane has better heat capacity and conductivity leading to better gas heat transfer. Catalyst temperature is said to be reduced by 7° C.    U.S. Pat. No. 4,642,360 teaches a method whereby inert catalyst support is used to preheat the incoming gas.
No prior art is known to have taught a way to deal with localized hot spots which develop when gas flow is reduced due to maldistribution of gas or excessive pressure drop in a particular tube, or when the catalyst is locally over active, or when local heat transfer is impaired. The problem of localized hot spots can be particularly troublesome if the heat transfer fluid degrades into carbon blocks. Accordingly, it would be desirable if there were available a reactor for vapor-phase fixed bed catalytic reactions with improved heat removal capability, suitable to maintain an essentially isothermal temperature profile throughout the length of the reactor regardless of the heat load in individual tubes. It would also be desirable if such a reactor were to be easy to construct, operate and maintain. It would further be desirable if the heat of reaction could be recovered to generate steam. Further, to facilitate the charging of new catalyst and the punch out of used catalyst, the reactor should employ vertical tubes with lengths not exceeding about 35 ft. to 50 ft.